The present invention generally relates to mechanical fastening and, more particularly, to installation of mechanical fastening systems such as nut, washer, and bolt combinations according to strict installation specifications.
In the manufacture of commercial and military aircraft, fastening systems that fasten various components of the airframe and aircraft structure may require the fasteners to be installed according to strict installation requirements designed to ensure structural efficiency and integrity. An example of such a fastening system is the familiar combination of a bolt, nut, and one or more washers, typically used to fasten two components with a clamping action. For certain types of applications, the term “pin” may be used synonymously for “bolt”. Setting the fasteners correctly is important.
To accomplish the installation of aerospace fastening systems according to the strict requirements designed to maximize structural efficiency of the hardware without compromising structural integrity, assembly of fastening systems is typically subject to three primary requirements: (1) no “threads in bearing”—meaning that only the unthreaded shank section of a bolt is allowed to contact the structure being clamped up; (2) no “shanking”—meaning that the nut cannot run so far down the threads such that the nut enters the thread transition zone of the bolt, where the bolt threads may be incomplete in the vicinity of the shank section of the bolt; and (3) sufficient “thread protrusion”—meaning that a predetermined amount of thread length must protrude completely through the nut to ensure complete nut engagement.
To comply with these, and other, requirements, aerospace fastening systems may be required to conform to installation specifications for various measurements. For example, a minimum pin protrusion dimension may be defined, and specific numerical values may be given for the dimension to exceed, according to the specific fastening system being installed, in order to guarantee compliance with requirement number (1) above.
FIGS. 1A and 1B show a typical fastening system 100 including a bolt 102 or pin 102, nut 104, and washer 106 combination. Bolt 102, nut 104, and washer 106 may be referred to generically as components of fastening system 100. Bolts 102 may be provided in various lengths—called the grip length of the bolt, which is related to the length of shank section 107 of bolt 102—depending on the thickness of structure 120 that is to be held together by the nut-bolt combination and through which the bolt 102 may pass. Length may be measured, for example, along longitudinal axis 101 of fastening system 100. Bolt 102 may include a threaded section having a thread length 108. The thread length 108 typically includes a thread transition zone 105 in the vicinity of shank section 107 where the threads of bolt 102 may be incompletely cut. The end 107a of shank section 107 is typically defined by a visible ridge at shank section end 107a between shank section 107 and thread length 108. The visible ridge may be used as an indication of the transition of shank section 107 into thread transition zone 105 or a boundary between thread transition zone 105 and shank section 107.
Different bolts may have a fixed thread length 108 for various grip lengths. In other words, a short bolt, such as bolt 102 shown in FIGS. 1A and 1B, may have threads cut for a certain length, the thread length 108, along the bolt from the end of the bolt, and the thread length 108 may be the same for a longer bolt and for a shorter bolt. Because the fasteners may have a fixed thread length 108 for various grip lengths, fasteners are required to conform to installation specifications regarding the height that the bolt is allowed to protrude, referred to as “protrusion”. More specifically, a minimum pin protrusion 116 dimension may be the specified minimum height that bolt 102 may protrude above the surface 118 of structure 120, which may be, for example, an aircraft structural component. Minimum pin protrusion 116 may also be referred to in the art as “minimum pin protrusion to avoid threads in bearing.” Bolt 102 not protruding far enough above the surface 118 to exceed minimum pin protrusion 116 may result in the threaded section of bolt 102 contacting the structure 120 being clamped up, exemplifying a fastening system that does not comply with the “no threads in bearing” requirement number (1) described above. A bolt 102 in which the threaded section contacts the structure 120 may result in improper fit of fastening system 100 to structure 120 as the threaded section of bolt 102 is typically smaller in diameter than the shank section 107 of bolt 102 for which structure 120 is drilled. Improper fit may lead to damage and possible failure of fastening system 100.
A maximum pin protrusion 110 dimension may be the specified maximum height that bolt 102 may protrude above the bearing surface 112 of nut 104. Maximum pin protrusion 110 may also be measured from the bearing surface 114 of washer 106, which is in contact with bearing surface 112 of nut 104. If the maximum pin protrusion 110 is exceeded, nut 104 could fully engage the last completely cut thread of the bolt 102 and enter the thread transition zone 105 before the required compression of the joint being fastened occurs. Nut 104 entering thread transition zone 105 on the bolt 102, or bindingly engaging incomplete threads of transition zone 105, which may be referred to as “bottoming out” on the threads of bolt 102, exemplifies a fastening system that does not comply with the “no shanking” requirement number (2) described above. A nut 104 that bottoms out, or engages the incompletely cut threads of transition zone 105, may result in damage to nut 104 and bolt 102, a false torque reading when tightening nut 104, or false clamp-up torque, and inadequate fastening of fastening system 100. Conforming to the maximum pin protrusion 110 specification will preclude, for example, having nut threads in the thread transition area 105 of the bolt shank and may ensure proper tightening of nut 104 and bolt 102 of fastening system 100.
A minimum thread protrusion 122 dimension may be the specified minimum height that bolt 102 may protrude above nut 104. Conforming to the minimum thread protrusion 122 specification may ensure, for example, that all threads common to the nut 104 and bolt 102 are engaged in order for fastening system 100 to function properly. For example, sufficient protrusion can allow for adequate nut retention due to full nut-to-bolt interference, which can act as an anti-back-off feature. Conversely, a thread protrusion that is less than the minimum thread protrusion 122 specification may cause fastening system 100 to fail. Bolt 102 not protruding far enough out of nut 104 to exceed minimum thread protrusion 122 exemplifies a fastening system that does not comply with the complete nut engagement requirement number (3) described above.
Two unknowns which complicate the washer selection process, i.e., determining the washer stack for a given fastening system installation, are: (1) the exact dimensions of the actual hardware involved (having non-zero tolerances) and (2) the structure thickness through which the bolt protrudes. Since the shoulder—such as the end of shank section 107 of bolt 102 at the transition to thread length 108—of the bolt must always protrude through the structure to avoid “threads in bearing”, one might think that one can determine the number of washers in the stack by using the thread length dimension and nut height. Each of those dimensions, however, has a tolerance which forces one to account for both extremes of thread length and nut height. So little thread length is available on some fastening systems that variation due to manufacturing tolerances alone precludes identification of set, or universal, washer stack-ups which will work for specific hardware. Therefore, no set washer combination can be defined which will always and simultaneously meet all three installation requirements. Thus, each installation may be regarded as a unique case.
Because each fastening system installation may practically be a unique case, ensuring that each fastening system complies with all three of the requirements described above may lead to trial and error methods to complete the assembly of the fastening system at each location. For example, in the absence of exact measurements of the particular fastening system being installed, a mechanic must sometimes use a trial and error process all the way through torque-up of the fastening system to determine if the selected washer stack-up meets requirements. With too small a stack of washers, the nut will engage the shoulder of the bolt below the threads (shanking), and with too large a stack, the nut will not be fully engaged with the bolt (insufficient thread protrusion through the top of the nut). Thus, a tool is needed that can help determine how a successful fastening system installation can be made without a trial and error process and prior to torque-up of the fastening system.
Currently, installation specifications monitor a minimum pin protrusion dimension—such as minimum pin protrusion 116 shown in FIG. 1A—to be sure that enough bolt is protruding through structure—such as structure 120—so that even with tolerances, the fastening system cannot have “threads in bearing”. A maximum pin protrusion dimension—such as maximum pin protrusion 110—is monitored to be sure that there is not so much thread length extending above the washers that the nut could have engaged incomplete threads near the shoulder below the threads (shanking). A minimum thread protrusion dimension—such as minimum thread protrusion 122—is monitored to be sure enough thread protrudes through the top of the nut to give confidence that the nut is fully engaged in fully formed threads and will not back off. For each type of fastening system, numerical specifications are given for each dimension, so that when the fastening system is within the numerical specification it is said to meet, or conform to, a dimensional requirement—such as minimum pin protrusion—for that type of fastening system. The values of the numerical specifications, i.e., the dimensional requirements, are set with regard to the tolerances so that when the dimensional requirements for minimum pin protrusion, maximum pin protrusion, and minimum thread protrusion are met, the fastening system will meet the qualitative requirements of having no “threads in bearing”, no shanking, and complete nut engagement, respectively.
As can be seen in FIG. 1A, the dimensions (minimum pin protrusion, maximum pin protrusion, and minimum thread protrusion) overlap and so are not independent of one another. In order to guarantee that a fastening system will meet the qualitative requirements (e.g. no “threads in bearing”), the numerical specifications for the dimensional requirements (e.g. minimum pin protrusion) must cover every possible case of tolerance variation, including the worst on worst tolerance cases—for example, a tallest nut within tolerances combined with a bolt having a shortest thread length within tolerances. Thus, it can be difficult to simultaneously meet all three dimensional requirements, particularly, for example, where a fastening system has limited thread length. Thread length must be sufficient for up to one wasted washer (where an additional washer was just barely needed), the tallest nut within tolerances, and the required thread protrusion. The dimensional requirement—for thread length, for example—is restrictive in the sense that a fastening system installation can be made that meets all the qualitative requirements without meeting all the dimensional requirements. Thus, the dimensional requirements are more restrictive than the qualitative requirements because of the need for the dimensional requirements to guarantee that the qualitative requirements are met over all tolerance cases.
Prior art measurement tools typically take some sort of numerical measurement of one of the installed fastening system dimensions—such as a pin protrusion dimension—and so are adaptable almost exclusively toward working in conjunction with dimensional requirements rather than qualitative requirements. If the fastening system 100 fails to meet any of the dimensional requirements, it is deemed not to conform to the installation specification, i.e., fastening system 100 does not meet the dimensional requirements. In such a case, the fastening system installation is assumed not to meet the qualitative requirements, and the installation is rejected. It is still possible in such a case, however, due to combinations of variation of the components within tolerances, as described above, for the installation to actually meet the qualitative requirements even though the dimensional requirements are not met. Thus, the prior art gages are generally not helpful in the efficient selection, without trial and error, of proper bolt, washer, and nut combinations for each unique case that arises out of each distinct location of a fastening system installation on a structure.
As can be seen, there is a need for a tool for installation of fastening systems—such as nut and bolt fastenings used on aircraft—according to strict installation specifications that are designed to meet both dimensional and qualitative requirements. There is also a need for a tool for installation of fastening systems that facilitates efficient selection, in a predictive manner avoiding trial and error, of a proper bolt, washer, and nut combination for each unique case of fastening system installation at distinct locations on a structure. Moreover, there is a need for an installation tool that provides a combination of measurements at one time, accounting for the interactions of tolerances between different fastening system dimensions whose specifications all need to be met simultaneously. Furthermore, there is a need for a fastening system installation system including an installation tool and a new type of installation specification that allows fastening system installations to be made directly according to qualitative requirements and that is less restrictive than prior art installation specifications limited to dimensional requirements.